This invention relates to a fuel injection control apparatus, which is used in diesel engines, for example, and which controls the amount of fuel injected in response to operating conditions of the engine.
A unit injector, which injects a controlled amount of fuel, is disclosed in U.S. Pat. No. 4,418,867. Fuel is supplied when an electromagnetic valve is energized, and the energizing current is supplied to the valve for a time period corresponding to the amount of fuel that is to be injected. When this controlled quantity of fuel is to be injected, the electromagnetic valve is operated again to close the compression pump chamber and the compression plunger of the pump operates to inject this controlled quantity of fuel.
With this kind of unit injector, however, the fuel overflow is returned to the fuel feed path, which results in pressure fluctuations in the feed path, and this in turn results in unstable operation of the fuel injectors provided for the other cylinders. Also, the back flow caused by the overflow port results in uneven fuel injection in the particular cylinder itself.
This electromagnetic valve is a three-way valve and the needle portion is subject to very high pressure, which causes problems with providing the device with sufficient mechanical strength. The overflow fuel is conducted to the nozzle spring chamber and a throttle is arranged in the feed path from the nozzle spring chamber to prevent an abrupt drop in overflow pressure so that an unnecessarily strong force acts on the nozzle needle, possibly damaging it.